1. Field of the Invention
The present invention relates to a transmission system for digital information such as digital image information having correlation among its codes.
2. Related Background Art
In a transmission system of the type that information signals such as image information signals are transmitted in digital form, a generally adopted method is to transmit digital signals by converting them into transmission codes matching transmission medium such as a communication pass and recording medium. In such a case, input data are often compressed prior to encoding due to restriction of transmission bit rate, and error correcting codes are added for dealing with transmission errors.
FIGS. 1A and 1B are block diagrams showing the schematic arrangement of a typical, conventional digital information transmission system, wherein FIG. 1A depicts a transmitting system and FIG. 1B depicts a receiving system. In the transmitting system, a compression encoding circuit 10 compression-encodes information codes such as digital information by using correlativity. An output from the compression encoding circuit 10 is applied to an error correction encoding circuit 12, in which it undergoes error correction encoding for dealing with transmission errors and is added with error-correcting codes. An output from the error correction encoding circuit 12 is applied to a modulation circuit which in turn modulates it in a form matching the characteristics of a transmission pass to which it is outputted.
In the receiving system on the other hand, a demodulation circuit 16 demodulates a transmitted signal from the transmission pass. An error correction decoding circuit 18 corrects errors generated on the transmission pass. An expansion circuit 20 performs expansion/decoding in the manner opposite to the compression encoding circuit 10. For those codes determined as not correctable by the error correction circuit 18 data replacement by modification such as by interpolation is conducted at an error modification circuit to modify errors of the codes.
However, mutual relationship among respective processes at the above-described conventional transmitting and receiving system has not been considered, and the overall system arrangement has been found still unsatisfactory.
An information unit with error detecting/correcting codes added as described above is called an error correction block. Conventionally, for processing information codes having both image codes and voice codes to be transmitted, two methods have generally been adopted. One method comprises providing different error correction blocks for image and voice codes as shown in FIGS. 2A and 2B, and using different circuits for the correction blocks for encoding and decoding processes. The other method comprises providing error correction blocks having the same code length for both image and voice codes, and using same circuits for the correction blocks. In FIG. 2, ID represents a block number and other additional codes, V an image code, A a voice code, and P an error detecting/correcting code.
With the former method of processing image and voice codes at different circuits, the circuit arrangement becomes bulky while wastefully using circuit portions which otherwise can be used in common. With the latter method of using the error correction blocks having the same code length, the information quantity per unit time is too different between image code and voice code so that, for example, errors which are a frifle for image information may become fatal to voice information to result in noticeable deterioration of voice information.